Photomasks having a metallized pattern on a light transmissive substrate are well known for their use in projection printing associated with the fabrication of semiconductor devices. The pattern on such a photomask has a high resolution metallic pattern with feature sizes rapidly approaching the 1 .mu.m to 2 .mu.m range. The quality of the metallized pattern is very critical to the semiconductor manufacturing process, and the photomasks are therefore carefully fabricated substantially free from any defects. Maintaining the photomasks in this defect free state is essential if acceptable device yields are to be achieved. In addition to particulate contamination, the various cleaning and handling operations that the photomask is subjected to can cause a slow deterioration of the pattern. When such defects are multiplied by the six or more masks in a set required to fabricate a semiconductor device the yield loss can become significant.
One approach to protecting the metallic photomask pattern from physical damage is described in U.S. Pat. No. 4,361,643 entitled "A Photomask and Method of Using Same" which issued to Banks et al. on Nov. 30, 1982 and is assigned to the instant assignee and is incorporated by reference herein. Banks et al. describe a technique whereby a planar, transparent coverplate is placed in intimate contact with the patterned surface of a transparent baseplate with an index matching material, having substantially the same index as both the coverplate and the baseplate, interposed therebetween. Such a mask not only protects the patterned substrate but dust or other particles depositing on the coverplate will be offset from the plane of the pattern and will not be focused on the photoresist coated substrate.
The Banks et al. technique has been found to be most effective. However, particles may become entrapped between the coverplate and the mask during the fabrication process. The only way these defects can be identified is to make actual prints with the same mask before and after the application of the coverplate thereon. Since unprotected masks can be damaged during such pre-protection runs the coverplate has been assembled thereto without obtaining prints prior to such assembly. Thus, undesirably, it is not known whether the integrity of the mask is compromised during the application of the coverplate.
Once such an uninspected mask is used in production and defects are noted the coverplate can be removed, the mask cleaned and the coverplate reapplied. However, such removal of the coverplate and cleaning of the viscous index matching fluid from the mask surface provides the potential to damage the metallic portion of the mask.
Accordingly, there is a need for a mask protection technique that provides particle offset associated with the coverplate along with mechanical protection during repair and reassembly while maintaining inspectability to qualify the mask for use.